Providing for an interaction between two mechanical devices requires a device for bringing about the interaction. Conventionally, such a device takes the form of a mechanical or hydraulic coupling. In the course of the development of electrical control systems, it is now also possible to bring about such an interaction in an electronic way, e.g., by transmitting control signals. For this purpose, an electrical control unit, or a so-called torque control element, is assigned to each of the two mechanical devices, by which the state of the mechanical device may be monitored and influenced using specific control measures.
The interaction between the mechanical devices may occur indirectly via the torque control elements that exchange data or signals regarding the state of the mechanical devices with each other.
Particularly in a motor vehicle, an interaction may be brought about in this manner. This may concern particularly the steering, that is, the transmission of a steering input from a driver with the aid of a steering handle to a vehicle wheel to be steered or a vehicle axle to be steered. Devices for transmitting steering inputs between the steering handle and the vehicle wheels to be steered or the vehicle wheel to be steered are called steer-by-wire systems. With these, it is possible to replace conventional steering devices such as steering rods and the like. For this purpose, steering inputs are transmitted with the aid of a computer, e.g., via a communication network of the motor vehicle.
In the event that the steer-by-wire system should fail, the steering capability must be maintained. This is feasible in that the normal operation of the steering provided by the steer-by-wire system is safeguarded by a mechanical backup for providing an emergency operation. In normal operation, the steering handle operated by a driver and the steered wheels of the vehicle are connected to each other via an electrical or electronic closed-loop control system. In emergency operation, the steering handle and the steered wheels of the vehicle are positively coupled via an electromechanical coupling.
Published German patent document DE 198 05 015 describes that in emergencies a mechanical positive coupling between the steering wheel and the steered vehicle wheels takes effect. For this purpose, a coupling, acted upon in a locking manner by spring force, is provided between the steering wheel and the steered vehicle wheels, which coupling may be opened by feeding hydraulic pressure into a displacement assembly. Due to its hydraulic interconnection, such a coupling has only limited applicability for a steer-by-wire system. The control of the hydraulic medium on the part of the steer-by-wire system would have to be provided in a roundabout way, for example, via an electronic wiring of a hydraulic circuit.
In the device described in published German patent document DE 100 46 168, a coupling is connected by two electric motors. To provide for a steer-by-wire operation, the electric motors keep the coupling in an open state. In predetermined failures, the coupling is closed automatically, it being possible, among other things, to provide for the electrical control motor, which is designed to continue to act upon steered vehicle wheels, to be switched off. In this type of control of a coupling, however, the functioning of the coupling is overly influenced by the functioning of the electric motors.
An objective of the present invention is to increase the operational reliability of a system for bringing about an interaction between two mechanical devices, e.g., in the case of a steering system for a motor vehicle having a steer-by-wire system.